This invention relates to an antilock control system and method for controlling vehicle wheel brakes.
When the brakes of a vehicle are applied, a braking force is generated between the wheel and the road surface that is dependent upon various parameters which include the road surface condition and the amount of slip between the wheel and the road surface. This braking force increases as slip increases until a critical slip value is surpassed. Beyond the critical value of slip, the braking force decreases and the wheel rapidly approaches lockup. Therefore, to achieve stable braking, an antilock braking control system (ABS) seeks to operate wheel slip at or near the critical slip value.
One known ABS is based on an electrically controlled fluid displacement piston. In this system, the brake pressure is modulated by means of a DC torque motor positioning a displacement piston in a cylinder hydraulically coupled to the wheel brake and whose volume is modulated to control the hydraulic pressure at the wheel brake. The motor is controlled to position the piston at an initial, fully extended home position at which a check valve is unseated by the piston to hydraulically couple the brake system master cylinder to the wheel brake to allow normal braking. When antilock brake pressure modulation is required, the motor retracts the piston (which allows the check valve to close to isolate the master cylinder from the wheel brake) to reduce brake pressure at the wheel brake and thereafter modulates the piston position to provide pressure control for antilock braking. When antilock braking is no longer required, the motor returns the piston to its extended home position at which the check valve is again engaged and unseated by the piston.
During ABS controlled braking, when an incipient wheel lock condition is sensed, an ABS cycle is initiated beginning with a pressure release phase wherein the motor current is controlled to quickly retract the piston to release brake pressure to allow recovery from the incipient wheel lock condition. When a recovery from the incipient wheel lock condition is sensed, a pressure apply phase is initiated in which the motor current is controlled to extend the piston to reapply brake pressure. During the apply phase, the motor current is ramped to ramp the brake pressure at a controlled rate in direction applying brake pressure until an incipient wheel lock condition is again sensed after which the ABS cycle is repeated. During the apply phase, the check valve functions to assure that brake pressure generated at the wheel brake does not exceed the master cylinder output pressure.
The antilock brake control system set forth above further typically includes a hydraulic passage in parallel with the check valve and which is controlled by an electromagnetic valve. During normal vehicle braking in which the check valve is unseated by the piston, the valve is open to provide a redundant direct communication between the brake system master cylinder and the wheel brake to assure integrity of the braking system. Upon entry into antilock controlled braking, the valve is operated to a closed position so that when the check valve is closed upon retraction of the piston, the brake system is isolated from the master cylinder as detailed above.
The ABS based on an electrically controlled displacement piston as set forth above is commonly referred to as a closed system in that after ABS operation is initiated, a fixed volume of fluid is used. This fixed quantity of fluid is moved by the displacement piston to and from the wheel brake to perform the ABS function. The maximum brake pressure that can be achieved in this system is the hydraulic pressure trapped when the check valve is closed at the beginning of ABS operation. If this trapped pressure is insufficient to initiate an incipient wheel lock condition during the apply phase of normal ABS operation, the piston will engage and open the check valve during the apply phase before an incipient wheel lock is initiated to begin a pressure release phase. The opening of the check valve couples the high pressure output of the master cylinder to the wheel brake resulting in an uncontrolled fluid flow into the wheel brake with an unpredicted rapid rise in brake pressure and wheel slip that is not a result of normal ABS operation.
There are various reasons that insufficient pressure may be captured in the braking system. First, insufficient pressure may be captured because of a high initial brake pressure apply rate which results in the initiation of ABS control of the front wheels before weight is transferred from the rear wheels to the front wheels. This results in a higher brake pressure required at the front wheel brakes during the apply phase to initiate the release phase of the ABS cycle. Second, insufficient pressure may be captured because of a decrease in braking efficiency as a result of brake heating during the braking period. Third, insufficient pressure may be captured because the coefficient of friction of the road surface increases after the ABS is first engaged so that a higher pressure is required during the apply phase to initiate an incipient wheel lock condition.